The cellular transcription factor HIF (Hypoxia Inducible Factor) occupies a central position in oxygen homeostasis in a wide range of organisms and is a key regulator of responses to hypoxia. The genes regulated by HIF transcriptional activity can play critical roles in angiogenesis, erythropoiesis, hemoglobin F production, energy metabolism, inflammation, vasomotor function, apoptosis and cellular proliferation. HIF can also play a role in cancer, in which it is commonly upregulated, and in the pathophysiological responses to ischemia and hypoxia.
The HIF transcriptional complex comprises an αβ heterodimer: HIF-β is a constitutive nuclear protein that dimerizes with oxygen-regulated HIF-α subunits. Oxygen regulation occurs through hydroxylation of the HIF-α subunits, which are then rapidly destroyed by the proteasome. In oxygenated cells, the von Hippel-Lindau tumor suppressor protein (pVHL) binds to hydroxylated HIF-α subunits, thereby promoting their ubiquitin dependent proteolysis. This process is suppressed under hypoxic conditions, stabilizing HIF-α and promoting transcriptional activation by the HIF αβ complex. See, e.g., U.S. Pat. No. 6,787,326.
Hydroxylation of HIF-α subunits can occur on proline and asparagine residues and can be mediated by a family of 2-oxoglutarate dependent enzymes. This family includes the HIF prolyl hydroxylase isozymes (PHDs), which hydroxylate Pro 402 and Pro 564 of human HIF1α, as well as Factor Inhibiting HIF (FIH), which hydroxylates Asn 803 of human HIF1α. Inhibition of FIH or the PHDs leads to HIF stabilization and transcriptional activation. See, e.g., Schofield and Ratcliffe, Nature Rev. Mol. Cell. Biol., Vol 5, pages 343-354 (2004).